Components that contain at least one organic semiconducting layer are considered to be organic semiconducting components. Known organic semiconductor components include, for example, organic light-emitting diodes (OLEDs), field-effect transistors, photodetectors and organic solar cells (OPVs), in which organic semiconducting materials are used, for example as charge transport material or blocker material, preferably as hole conductors or electron blockers.
By way of example, organic light-emitting diodes utilise the property of materials to emit light when, by application of a voltage, suitable charge carriers are formed, which form excited states with recombination thereof and in turn transfer into the ground state with emission of light. To improve the efficiency of organic light-emitting diodes, these often have charge transport layers besides the actual emitting layer, which charge transport layers are responsible for the transport of negative and positive charge carriers in the emitting layer. These charge transport layers are divided into hole conductors and electron conductors depending on the type of transported charge carriers. A similar layer structure is also known for photovoltaic components, such as organic solar cells. Organic semiconducting components with a number of layers can be produced by known methods, such as vacuum evaporation or the deposition from a solution.
A desired property of organic semiconducting components is a high conductivity. This can be improved, for example, by doping individual layers of the organic semiconducting components. As a result of the doping, the conductivity of the layer increases and one problem concerning a low charge carrier movability is thus overcome.
It is known to change the electrical properties, in particular the electrical conductivity, of organic semiconductors by means of doping, as is also the case with inorganic semiconductors (silicon semiconductors).
Here, an increase of the initially rather low conductivity, and also a change in the Fermi level of the semiconductor depending on the type of dopant used, are achieved by generating charge carriers in the matrix material. Here, doping leads to an increase of the conductivity of charge transport layers, whereby ohmic losses are reduced and an improved transfer of the charge carriers between contact and organic layer is achieved. The doping is characterised by a charge transfer of dopant to a close matrix molecule (n-doping, electron conductivity increases) or by a transfer of an electron from a matrix molecule to a close dopant (p-doping, hole conductivity increases). The charge transfer can be incomplete or complete and can be determined for example by the interpretation of vibration bands from FTIR measurements.
The conductivity of a thin-film sample can be measured using what is known as the two-point method. Here, contacts made of a conductive material, for example gold or indium tin oxide, are applied to a substrate. The thin film to be examined is then applied to the substrate over a large area, such that the contacts are covered by the thin film. After applying a voltage to the contacts, the current then flowing is measured. From the geometry of the contacts and the layer thickness of the sample, the conductivity of the thin-film material is given from the resistance thus determined.
At the operating temperature of a component with doped layer, the conductivity of the doped layer is to exceed the conductivity of the undoped layer. To this end, the conductivity of the doped layers at room temperature is to be high, in particular greater than 1.10-8 S/cm, but preferably in the range between 10-6 S/cm and 10-2 S/cm. Undoped layers have conductivities of less than 1.10-8 S/cm, usually less than 1.10-10 S/cm.
A further essential property of the materials used in a component is the thermal stability thereof. This is of particular importance when the component is produced by vacuum vapour deposition.
The temperature stability can be determined using the same method or using the same construction by heating the (undoped or doped) layer gradually and measuring the conductivity after a rest period. The maximum temperature that the layer can withstand without losing the desired semiconductor property is then the temperature immediately before the conductivity nosedives. For example, a doped layer on a substrate with two adjacent electrodes, as described above, can be heated in steps of 1° C., wherein a period of 10 seconds elapses after each step. The conductivity is then measured. The conductivity changes with temperature and nosedives abruptly from a certain temperature. The temperature stability thus indicates the temperature up to which the conductivity does not nosedive abruptly.
With these methods, it should be ensured that the matrix materials have a sufficiently high purity. Such purities can be achieved with conventional methods, preferably gradient sublimation.
The properties of the different materials involved can be described by the energy loads of the lowest unoccupied molecular orbital (LUMO for short; synonymous with: electron affinity) and of the highest occupied molecular orbital (HOMO for short; synonymous with: ionisation potential).
A method for determining ionisation potentials (IP) is ultra-violet photoelectron spectroscopy (UPS). Ionisation potentials for the solid body are generally determined, however it is also possible to measure possible ionisation potentials in the gas phase. Both variables differ by solid body effects, such as the polarisation energy of the holes that are produced in the photoionisation process (N. Sato et al., J. Chem. Soc. Faraday Trans. 2, 77, 1621 (1981)). A typical value for the polarisation energy is approximately 1 eV, however greater deviations may also occur.
Here, the ionisation potential relates to the start of the photoemission spectrum in the region of the high kinetic energies of the photoelectrons, that is to say the energy of the most weakly bonded photoelectrons.
An associated method (inverse photoelectron spectroscopy (IPES)) can be used to determine electron affinities (EA). However, this method is less widespread. Alternatively, solid body energy levels can be determined by electrochemical measurement of oxidation potentials (Eox) and/or reduction potentials (Ered) in solution. A suitable method is cyclic voltammetry (CV). Empirical methods for deriving the solid body ionisation potential from the electrochemical oxidation potential are described in the literature (for example B. W. Andrade et al., Org. Electron. 6, 11 (2005); J. Amer. Chem. Soc. 127, (2005), 7227.).
No empirical formulas are known for the conversion of reduction potentials into electron affinities. This lies in the difficulty of determining electron affinities. A simple rule is therefore often applied: IP=4.8 eV+e·Eox (vs. ferrocene/ferrocenium) or EA=4.8 eV+e·Ered (vs. ferrocene/ferrocenium), wherein e means the electron charge (see B. W. Andrade, Org. Electron. 6, 11 (2005) and Ref. 25-28 therein). For the case that other reference electrodes or redox pairs are used to reference the electrochemical potentials, methods for the conversion are known (see A. J. Bard, L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2nd edition 2000). Information concerning the influence of a solvent can be found in N. G. Connelly et al., Chem. Rev. 96, 877 (1996).
It is usual, although also not absolutely correct, to use the terms “energy of the HOMO” E(HOMO) and “energy of the LUMO” E(LUMO) synonymously with the terms ionisation energy and electron affinity respectively (Koopmans Theorem). Here, it should be noted that the ionisation potentials and electron affinities are given such that a higher value means a stronger binding of a liberated or accumulated electron. The energy scale of the molecular orbitals (HOMO, LUMO) is the opposite. The following is therefore true for a rough approximation: IP=−E(HOMO) and EA=−E(LUMO).
In order to record a cyclic voltammogram, the substance to be examined is provided in an electrochemical cell with working electrode, counter electrode and reference electrode together with a conductive salt (for example tetrabutylammonium hexafluorophosphate, TBAPF6) and a solvent (for example dichloromethane (DCM), tetrahydrofuran (THF)). A voltage cycle is then applied to the working electrode (for example 0.0 V→1.6 V→−2.0 V→0.0 V) and is run through at a feed rate (for example 100 mV/s). Oxidative and reductive processes are made noticeable by a rise of the current. Here, in the case of reversible processes, a corresponding reductive process also takes place with each oxidative process. The redox potential is calculated here from the mean value of the peak points. In the case of irreversible processes, the peak onset is used.
EP 2042481 A1 discloses aromatic amine terphenyl derivatives that can be used in organic electroluminescence components.
EP1995234A1 describes mixtures of p-terphenyl compounds and electrophotographic photoreceptors produced with use of these mixtures.
The matrix materials known in the prior art for use in organic semiconducting components can be improved further still in terms of their conductivity, their thermal stability and also in terms of their processability from a solution.
One object of the present invention is therefore to overcome the disadvantages from the prior art and to provide materials that lead to improved organic semiconducting components, which in particular demonstrate improved conductivity, are thermally stable and/or can be processed easily from a solution. In addition, materials that can be produced easily and cost-effectively in high purity are desirable.
A further object of the present invention is to provide corresponding organic semiconducting components.